they both can't know, according the the opening only one knows. I don't think they would have made it that obvivous sp?
The opening says "only one knows the truth" but they don't specify WHAT truth.
What we DO know:
(1) EQ/Regina definitely knows who she is in both worlds. I'm pretty sure Regina knows the Storybrooke and fairy tale identities of everyone.
(2) Snow/Mary Margaret, Jiminy/Archie, and the rest of the people of Storybrooke don't remember their fairy tale lives. (Except for poor Sheriff Graham who started to remember and is now dead.)
(3) Somehow Henry (who, so far as we know, did not have a prior life in fairy tale world) thinks he can match up the Storybrooke people with their fairy tale identities
(4) Emma is Snow & Prince Charming's daughter, who escaped the curse via the tree/cabinet
(5) Rumpel/Mr. Gold also knows who he is in both worlds, AND knows the Storybrooke and fairy tale identities of everyone. (I'm guessing this by the things he has in his shop and the things he's said to some of the Storybrooke residents)
(6) Since Rumpel knows that Snow's daughter is named Emma, and Rumpel is the one who told Snow that her daughter would come back to break the curse on her 28th birthday, I'm 99.9% sure that Rumpel knows that Emma IS Snow's daughter.
(7) EQ/Regina doesn't know what happened to Snow's daughter; hence, she doesn't know that Emma is Snow's daughter. I think this is the one fact that Rumple/Gold knows that EQ/Regina doesn't.
So I'm thinking that when the opening says "only one knows the truth" they mean that only Rumple/Gold knows the truth about Emma being Snow's daughter and the one who can break the curse. Regina doesn't know Emma's true identity - she just sees her as competition for Henry (and I guess competition for Sheriff Graham).